There is growing interest in the possibility that drugs of abuse subvert normal learning and decision- making processes, leading to the development of compulsive, pathological drug-seeking behavior. Chronic exposure to psychostimulants, like cocaine, results in sensitization of nigrostriatal and mesolimbic dopamine pathways, which are critically involved in the acquisition of habits (i.e., rigid, stimulus-bound responses) and the expression of incentive motivation (i.e., the capacity for a reward-related cue to facilitate appetitive behaviors, or induce 'wanting'). In this project, we will evaluate two distinct (but not mutually exclusive) theories of addiction: that drug exposure allows habits to dominate the control of action selection (at the expense of deliberative, goal-directed action selection), and that drug exposure potentiates the influence of reward-related environmental cues over performance. Our plan is to use a combination of carefully controlled behavioral procedures and well-established neurochemical detection techniques to answer the following questions: (i) Does dopamine release during instrumental training match the profile of the prediction error-based reinforcement signal assumed to be responsible for governing habit formation? (ii) What are the effects of cocaine pre-exposure on the acquisition and expression of habitual performance and on reinforcement-related dopamine signaling? (iii) Does mesolimbic dopamine efflux mediate the incentive motivational processes that guide cue-based action selection? (iv) and, What are the effects of repeated cocaine exposure on cue-based action selection and on the mesolimbic dopamine response to reward-related cues? Recent studies have shown that response-contingent cocaine elicits stronger neurochemical effects and results in more dramatic and longer-lasting changes in the circuitry underlying dopamine release than noncontingent cocaine. Such findings raise questions about the validity of studies assessing the effects of experimenter-administered drugs on learning, behavioral control, and brain chemistry. Therefore, a secondary objective of the current application is to determine whether the behavioral and neurochemical effects of repeated cocaine exposure depend on the mode of drug delivery.